When a system of conduits is operated under positive or negative pressure, leaks in the conduits create acoustic vibrations in the ultrasonic range, i.e. between 20 KHZ and 100 KHZ. It is known that these leaks can be detected with ultrasonic transducers. For example, U.S. Pat. No. 4,416,145 of Goodman et al., which is owned by the assignee of the present invention, describes apparatus for detecting such leaks.
The ultrasonic vibrations produced by a leak are directional and are typically detected by passing a detector unit about the system of conduits. This unit has a highly directional transducer and a preamplifier so that the location and intensity of even small leaks is easily determined. Also the ultrasonic frequency of the leak is usually outside the frequency range of ambient noise so that the signal from a leak can be separated from most of the competing noise by selective filtering.
One problem with prior ultrasonic detectors is that the preamplifiers must have a very wide dynamic range in order to detect small leaks without becoming saturated by sound from large leaks. When the ultrasonic detector is carried about in a search for leaks, the operator can manually change the dynamic range in order to measure the intensity of sound from both large and small leaks.
In some situations continuous monitoring of the integrity of a conduit system is important, and even critical e.g. in a chemical plant where hazardous gases are created or used. Since it would be impractical to have an operator standing at critical points waiting for months or years to detect a leak that may never come, it would be advantageous to have remote ultrasonic detectors located at various critical points in the system and to have the signals from these detectors delivered to a central control device. This central control device may be under direct operator supervision or under computer control.
In industrial environments where remote ultrasonic detection may be advantageous, there are likely to be strong electrostatic and electromagnetic signals which can interfere with control signals to the detectors and measured signals from the detectors. Since it is also likely that there will be great distances between the measurement locations and the central control location, the transmission system must have a high immunity to interference. Further, the cables should be inexpensive in order to keep the cost of the overall ultrasonic detector system to a minimum.
A standard means of providing low cost immune transmission is the so-called "current loop." In such a system, analog as well as digital signals are transmitted as current signals, as opposed to voltage signals. Industry standards specify ranges of 4-20 milliamps or 10-50 milliamps from zero to full scale for such a system.
It may be necessary to locate remote transducers in harsh environments, even out of doors. Thus, it would be advantageous if the transducer could be made resistant to adverse water, wind and temperature conditions, without seriously degrading its ability to detect ultrasonic vibrations.